The present invention relates to night vision systems, and more particularly, to a system and method of controlling operation of an active night vision system.
Night vision systems allow a vehicle occupant to better see objects during relatively low visible light level conditions, such as at nighttime. Night vision systems typically are classified as either passive night vision systems or active night vision systems. Passive systems simply detect ambient infrared light emitted from the objects within a particular environment. Active systems utilize a light source to illuminate a target area and subsequently detect the infrared light reflected off objects within that area.
Passive systems typically use far-infrared cameras that are characterized by low resolution and a relatively narrow field-of-view. Such cameras must be located on the vehicle exterior in order to acquire requisite infrared energy in the operating environment. Externally mounted cameras can negatively affect vehicle styling. Far-infrared cameras are also costly to manufacture and generate images that have poor contrast, which can be difficult to interpret.
Active systems provide improved resolution and image clarity over passive systems. Active systems utilize laser or incandescent light sources to generate an illumination beam having near infrared light energy, and charged coupled devices or CMOS cameras to detect the reflected infrared light. Active systems commonly deploy a light source external to the vehicle to transmit a significant amount of light energy and provide a bright scene for imaging.
Diode lasers are preferred over incandescent light sources for several reasons. Incandescent light sources are not monochromatic like diode lasers, but instead emit energy across a large spectrum, which must be filtered to prevent glare onto oncoming vehicles. Filtering a significant portion of the energy generated from a bulb is expensive, energy inefficient, and generates undesired thermal energy. Also, filter positioning is limited in incandescent applications, since the filter must be located proximate an associated light source. As well, multiple incandescent sources are often required to provide requisite illumination, thus increasing complexity and costs.
Although diode lasers are preferred over incandescent light sources, their emission wavelength is dependent on temperature, such that the output wavelength of a diode laser shifts approximately 0.25 nm for every one-degree Celsius change in temperature. The temperature sensitivity is especially evident when the laser diodes are externally mounted on a vehicle, since external temperatures vary considerably.
There is a current desire to utilize laser based active night vision systems within automotive vehicles. This utilization is limited by current and upcoming operating safety standards of laser products. These standards include acceptable operating levels of illumination, laser intensity levels, maximum permissible exposures, accessible emission limits, illumination distances and height requirements, as well as other known standards and requirements. Examples of laser product operating safety standards can be found in the European and international laser safety standard IEC 60825-1 from the International Electrotechnical Commission.
Thus, there exists a need for an improved system and method of controlling operation of a laser based active night vision system that satisfies safety-operating standards of interest and maintains a desired wavelength operating range of a light source of the night vision system.